3.1 Introduction
Automobile was invented more than 100 years ago. As
an important part of automobile, automobile steering system was created with
automobile evolution in the configuration, function and appearance aspect. Steering
systems is basically a mechanical linkage or mechanism through which driver can
controls and operate the movement or direction of an automobile. Along with the
development of automobile technique and performance itself, automobile steering
system needs to be innovated by increasing system dynamization and controllability
step by step. In order to be adapt to the mutative environments, it comes to
the point for the steering system that how to increase its flexibility and changeability.
In early times when automobile was just
invented, the driver turned a handle or a rail with a steering shaft in his
hand to drive. The automobile had a pinion-rack steering gearing, which was the
earliest ancestor of automobile steering gearing. The ratio of steering gearing
was 1 to 1. It needed much power to drive. When automobile pre-positive engine
were developed in 1891, the weight of the head of automobile was increased,
thus steering system vibration might be transferred to the driver by turning handle
or rail, and it made the driver feel too tired. As a single-hinge mechanism,
the original handle or rail with a steering shaft should be improved, and its
flexibility should be increased to avoid the disadvantage. In early times, the
systems of handle or rail with a steering shaft are all simple, and they only
have the function of turning right or left. Later steering wheel was applied in
automobiles. After 1990, the functions of steering wheel trend to diversification.
Some accessional functions are added where the advantage of the multi-function
steering wheel is that drivers needn’t to move in so much room, and they can
control the automobile conveniently for many functions. From the handle or rail
steering system to multi-function steering wheel, and then developing to
conceptional system without steering wheel, it is right a process from simple
system to complex system, and then developing to simple system.
3.2 Functions of Steering System:
The functions of steering system are; to allow swinging the wheels to the left or right for turning the vehicle at the will of the driver (allow driver to guide the vehicle). It converts the rotary movement of the steering wheel into an angular turn of the front wheels. It provides directional stability with the help of proper steering geometry. It is helps to minimize the tyre wear by making proper road contact. It optimizes tyre life also. It multiplies the effort of the driver by act as leverage in order to make it fairly easy to turn the wheels. It helps in achieving self-centering efforts with maintain correct steering mechanisms. It absorbs a major part of the road shocks thereby preventing them to get transmitted to the hands of the driver without much stress on him.
3.3 Requirement of Steering System:
1. Properly designed steering system, works well and guides a vehicle to move in the correct direction.
2. It must satisfy the ackerman’s condition.
3. When the steering wheel is released, the wheels must return automatically to the straight-ahead position and must remain stable in this position.
4. The steering should have as low ratio as possible in order to obtain ease of handling.
Steering mechanism should be very accurate, easy to install and handle or driver’s efforts should be minimum to steer.
5. It should provide maximum directional stability to vehicle. The road shocks by the wheels are not transmitted to driver hands; it should absorb the road shocks.
6. It must keep the wheel at all times in to rolling motion without rubbing on the road.
7. This system should associate to control the speed.
8. It must light and stable. It must easily be operated with less maintenance.
9. It should have self-centering action to some extent.
10. The steering system should not be affected by the side thrusts, cornering forces and wind effects.
11. A good steering mechanism is must designed in a manner so that it will not permit lateral slip of front wheels during steering.
12. There must be true rolling of wheels at the time of steering. The front wheels are to allow their left and right swing for steering the vehicle.
13. It should be certain degree irreversible so that the shocks of the roads surface are not transmitted to the hands of the driver.
1. Properly designed steering system, works well and guides a vehicle to move in the correct direction.
2. It must satisfy the ackerman’s condition.
3. When the steering wheel is released, the wheels must return automatically to the straight-ahead position and must remain stable in this position.
4. The steering should have as low ratio as possible in order to obtain ease of handling.
Steering mechanism should be very accurate, easy to install and handle or driver’s efforts should be minimum to steer.
5. It should provide maximum directional stability to vehicle. The road shocks by the wheels are not transmitted to driver hands; it should absorb the road shocks.
6. It must keep the wheel at all times in to rolling motion without rubbing on the road.
7. This system should associate to control the speed.
8. It must light and stable. It must easily be operated with less maintenance.
9. It should have self-centering action to some extent.
10. The steering system should not be affected by the side thrusts, cornering forces and wind effects.
11. A good steering mechanism is must designed in a manner so that it will not permit lateral slip of front wheels during steering.
12. There must be true rolling of wheels at the time of steering. The front wheels are to allow their left and right swing for steering the vehicle.
13. It should be certain degree irreversible so that the shocks of the roads surface are not transmitted to the hands of the driver.
3.4 Principle of Correct Steering:
Automobile steering linkage mechanism is based on “Ackerman’s
Steering Principle” while "Principle of Correct Steering" is the
backbone of that linkage. In the mechanism consists of a cross link connected
to short axles and front wheels through short arms. These form the bell crank
lever. In case of straight motion of automobile the cross-link remains parallel
to short links both makes angle α from the horizontal axis of chassis. This
system provides the directional change in the movement of an automobile and
maintain in a position as per the driver’s decision without much strain on him.
An elegant and simple mechanism to approximate ideal steering was patented at England in 1818 by Rudolph Ackerman, and though it is named after him, the
actual inventor was a German carriage builder called Georg Lankensperger who
designed it two years earlier.
The correct steering is achieved when all the
four wheels are rolling perfectly under all conditions of running. While taking
turns (fig- 3.1) condition of perfect rolling is satisfied if the axes of the
front wheels when produced meet the rear wheel axis at one point. Then this point
is the instantaneous centre (I) of the steering geometry. It is seen that the
inside wheel is required to turn through a greater angle than the outer wheel.
The larger the steering angle, the smaller is the turning circle. There is, however,
a maximum to which we can go as regards the steering angle. It has been found
that the steering angle (of the inner wheel) can have a maximum value about
44º. The extreme positions on either side are called lock positions. The
diameter of smallest circle which the outer front wheel of the car can transfer
and obtained when the wheels are at their extreme positions is known as the
turning circle.
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Fig 3.1: Principle of correct steering
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For correct steering,
cot φ = y+c / b, (φ=angle of inside lock)
cot φ = y+c / b, (φ=angle of inside lock)
= y/b + c/b
= cotθ + c/b, (θ= angle of outside lock)
cot φ ₋ cotθ =
c / b…. (equation of
correct steering).
Above equation represents the basic condition for
the steering mechanism for perfect rolling of all wheels.
The condition for correct steering is that all the four wheels must rotate about the same instantaneous center I which lies on the axis of the back wheels. The axis of the inner wheels makes a larger angle θ than the angle φ subtended by the axis of outer wheel; this is the “Principle of Correct Steering” mechanism. And that principle follows much better known “Ackerman’s Steering Geometry”. The term “steering geometry” (also known as “front-end geometry”) refers to the angular Relationship between suspension and steering parts, front wheels, and the road surface. Because alignment deals with angles and affects steering, the method of describing alignment measurements is called steering geometry. There are five steering geometry angles: Camber , Caster ,Toe, Steering axis inclination, and Toe-out on turns. The intention of ackerman’s geometry is to avoid the need for tyres to slip sideways when following the path around a curve. And behind the same curve path, both rear wheels are also rotate at a slightly different speed from each other to satisfy the correct steering condition, so that the ackerman principle works perfectly. The transition of the rear wheels and their regulations are maintained by the ‘differential unit’, which is plays a vital but hidden role in the correct steering mechanism. And therefor it is the most hidden component for steering geometry from final drive line system. A front wheels curve path line and center line of rear wheel axles is always cuts at one common point, called 'I' (instantaneous centre), and these is only possible with the proper functions of steering & differential.
3.5 General arrangement of Steering System:
The condition for correct steering is that all the four wheels must rotate about the same instantaneous center I which lies on the axis of the back wheels. The axis of the inner wheels makes a larger angle θ than the angle φ subtended by the axis of outer wheel; this is the “Principle of Correct Steering” mechanism. And that principle follows much better known “Ackerman’s Steering Geometry”. The term “steering geometry” (also known as “front-end geometry”) refers to the angular Relationship between suspension and steering parts, front wheels, and the road surface. Because alignment deals with angles and affects steering, the method of describing alignment measurements is called steering geometry. There are five steering geometry angles: Camber , Caster ,Toe, Steering axis inclination, and Toe-out on turns. The intention of ackerman’s geometry is to avoid the need for tyres to slip sideways when following the path around a curve. And behind the same curve path, both rear wheels are also rotate at a slightly different speed from each other to satisfy the correct steering condition, so that the ackerman principle works perfectly. The transition of the rear wheels and their regulations are maintained by the ‘differential unit’, which is plays a vital but hidden role in the correct steering mechanism. And therefor it is the most hidden component for steering geometry from final drive line system. A front wheels curve path line and center line of rear wheel axles is always cuts at one common point, called 'I' (instantaneous centre), and these is only possible with the proper functions of steering & differential.
3.5 General arrangement of Steering System:
In the evolution of automobile steering system, two
front wheels substitute shaft drive is applied firstly; pinion-rack steering
gearing design went to theorization later, and it was processed precisely, thus
pinion-rack steering gearing is applied in automobiles formally. With
automobile techniques improving themselves, automobile steering system also
developed rapidly. There was a tie-rod which connected the attachments of
steering shaft one another. A special steering rack controlled the tie-rod
moving to the right and left, and made the inner wheel and outer wheel turn.
The mechanism with a gear reducer made the automobile system more flexible to
control, and insulated vibration. The handle and rail were replaced by circular
steering wheel. And the circular steering wheel with a steering shaft moved to
the right or left. The former vertical fixing had developed to gradient fixing
for both steering wheel and steering shaft.
In any motor vehicles, steering is main component. Mainly steering is linked to the front axles with gear train mechanism. On the front axle, wheels are mounted, and with the help of steering wheel, the driver can turn the vehicle in right, left or straight directions. A general arrangement of automobile steering system is steering wheel, steering column & its shaft, steering gear box and steering linkages. And it has been shown in the Figure 3.2.
In any motor vehicles, steering is main component. Mainly steering is linked to the front axles with gear train mechanism. On the front axle, wheels are mounted, and with the help of steering wheel, the driver can turn the vehicle in right, left or straight directions. A general arrangement of automobile steering system is steering wheel, steering column & its shaft, steering gear box and steering linkages. And it has been shown in the Figure 3.2.
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Fig 3.2 General arrangement of automobile steering system
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1. Steering wheel: A wheel turned by the driver of a
motor vehicle, ship, etc, when he or she wants to change the direction of the
vehicle. In early times, there were two kinds of appearances of automobile
control mechanism. They were simple handle and two rails crossed. The ideas
came from the steering wheel of steamboat. After 1986, the appearance of
automobile control mechanism was basically finalized for the steering wheel.
2. Steering column & shaft:
3. Steering gear: The gear mounted on the lower end
of the steering column, that are used to multiply the driver turning force. It
converts the rotary motion of the steering wheel into the straight line motion.
4. Pitman arm: Transfers gearbox motion to the
steering linkage. Pitman arm is splinted to the gearbox.
5. Steering linkage: The system that connects the
steering wheel to the front wheel and allows the wheel to change direction in
response to command from driver. Steering linkage is the arrangement of
different links namely; pitman arm (drag arm), drag link /center link /push or
pull rod, tie rods and steering knuckle /steering arm.
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